CN1108850C - Method for removing nitrogen oxides using natural manganese ores - Google Patents

Abstract

There is disclosed a method for removing nitrogen oxides of exhaust gas using natural manganese ores. In the method, ammonia is used as a reductant to selectively reduce the nitrogen oxides in the presence of a catalyst prepared from the natural manganese ores. The catalyst allows nitrogen oxides to be completely removed from exhaust gas at a relatively low temperature of 130-250 DEG C without discharging unreacted ammonia. The catalyst is superior in economical terms in addition to preventing the deleterious effects which occur when discharging ammonia.

Description

Method for removing nitrogen oxides with natural manganese ore

technical field

In general, the present invention relates to removal methods of nitrogen oxides (hereinafter "NOx"). More specifically, the present invention relates to the use of natural manganese ore as a catalyst for selectively reducing NOx contained in exhaust gas.

Background technique

Many methods have been proposed for removing NOx from exhaust gases (from burners, boilers, etc.). From economic and technical considerations, the Selective Catalytic Reduction (SCR) technology has been evaluated as the most preferred technology, and the principle of this technology has been deeply studied. In this SRC technology, NOx (such as nitrogen monoxide and nitrogen dioxide) can be reduced to nitrogen and water using ammonia as a reducing agent in the presence of a catalyst, as shown in equations I-IV:

(I)

(II)

(III)

(IV)

The success of SRC technology depends on the catalyst.

The general characteristic of catalysts used in SRC technology is that the higher the reaction temperature, the higher the conversion rate of NOx. The temperature at which NOx conversion reaches its maximum value varies depending on the catalyst type, and it is an inherent property of each catalyst. However, ammonia is easily oxidized at high temperature by reacting with oxygen contained in exhaust gas, thereby losing its function as a reducing agent, as shown in reaction equations V and VI:

(V)

(VI)

Various components are present in most NOx-containing exhaust gases which can seriously affect SRC technology. For example, oxygen, water vapor, and sulfur oxides have a great influence on the activity of the catalyst. In addition, a part of the ammonia used to remove NOx may not react and may cause environmental pollution when discharged together with exhaust gas. In this case, the supply of ammonia must be controlled, or the unreacted ammonia must be treated by an oxidation reaction before being released into the air.

There are many catalysts available for SRC technology. In the case of noble metal catalysts, they have been reported to be unstable to sulfur dioxide poisons, with the result that they lose the vast majority of their catalytic activity within 40 minutes of initiation of the reaction (Foley, JM, Katzer, JR and Monogue, WH: Ind. Eng. Chem. Prod. . Res. Dev., 18, 170 (1979)). As for V ₂ O ₅ catalysts, they are generally impregnated in SiO ₂ , Al ₂ O ₃ or TiO ₂ , and it is reported that it shows excellent selective catalytic reaction effect at around 300°C (Barten, H., Janssen, FJJG, Van den Kernhof, F, MG, Leferink, R., Vogt, ETC, Van Diller, AJ and Geus, JW: "Preparative reactions over group IV catalysts" (B. Delmon, P. Grange, PA, Jacobs and G. Poncelet Eds), Elsevier, Amsterdam, 103 (1987)). Zeolite catalysts, usually impregnated in Cr, Fe or Cu salts, have been reported to show excellent NOx scavenging performance over a wide temperature range up to 500 °C (Karlesson, HT and Rosenberg, HS: Ind. Eng. Chem. Prod. Res . Dev., 23, (1984)). As mentioned above, intensive research and Schinler's work have been carried out in the preparation of catalysts for NOx removal.

For manganese catalysts, Miyazaki, Kazuhide, T. US Patent 3,975,498 discloses electrolytic manganese dioxide for removal of Nox by adsorption.

US Patent 4,883,647 discloses the use of manganese nodules for the removal of at least one pollutant contained in exhaust gases. Like natural manganese ores, manganese nodules contain iron, manganese, silicon, calcium and phosphorus. However, in the state of manganese, manganese nodules are different from natural manganese ores. To be more precise, manganese nodules contain 15-30% by weight of manganese and traces of platinum, nickel, cobalt, copper, titanium and lead, and manganese exists in crystalline form, while in manganese ore manganese exists in oxide form . Manganese nodules are also different from natural manganese ores in terms of state of existence, place of origin, manganese content and physical properties. The chemical composition and physical properties of the manganese nodules are listed in Table 1 below.

Table 1

Chemical composition and physical properties of manganese nodules Chemical Composition (% by weight) mn 11-39 Si 1-10 P 0.5-6 Fe 6-23 Ca 0.5-13 Density(kg/ ^m3 ) 1640 Surface area ( ^m2 /g) 140

US Patent 4,883,647 to Kainer, H., Grimm, D., Schnelle, W and Haibach, P also discloses manganese nodules together with ammonia (used as a reducing agent) to scavenge NOx. The patent provides data for NOx conversion of 30-50% in the temperature range of 250-350°C. However, the conversion is too low and the treatment temperature is too high.

disclosure of invention

The inventors of the present invention have made extensive and intensive studies on the selective removal of NOx contained in exhaust gas and found that natural manganese ore exhibits excellent NOx catalytic reduction activity at low temperature, and it is not necessary to carry out difficult and costly processing of the ore. processing.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to overcome the above-mentioned problems encountered in the prior art and to provide a method for removing NOx from exhaust gas. In this method, NOx contained in exhaust gas can be efficiently reduced at a relatively low temperature.

Another object of the present invention is to provide a more economical SCR method for removing NOx in exhaust gas.

According to the present invention, the above object can be achieved by providing a method for removing nitrogen oxides in exhaust gas. In said method, the selective catalytic reduction technique of ammonia as reducing agent is carried out in the presence of a catalyst made from natural manganese ore.

Brief introduction to the drawings

The above and other objects and aspects of the present invention will be apparent by describing the embodiments of the present invention with reference to the accompanying drawings. in,

Fig. 1 represents the variation of the conversion rate of NO contained in the exhaust gas with temperature when the exhaust gas is treated with ammonia in the presence of the catalyst prepared from natural manganese ore according to the method described in Example 1, and shows that nitrogen dioxide in the exhaust gas after treatment and ammonia emissions.

Figure 2 shows the graph of the NOx conversion rate versus the _O2 concentration in the exhaust gas at different temperatures (175°C and 200°C) when using the catalyst prepared in Example 1.

Figure 3 shows a plot of NOx conversion as a function of space velocity (GHSV) versus temperature when using the catalyst prepared in Example I.

Figure 4 shows the variation of the conversion rate of NOx contained in the exhaust gas as a function of temperature when the exhaust gas is treated with ammonia in the presence of a catalyst obtained from natural manganese ore according to the method described in Example II, and shows the emission of ammonia in the exhaust gas after treatment quantity.

Fig. 5 is a graph showing the NOx conversion rate versus temperature for different concentrations of the natural manganese ore component concentration contained in the catalyst in Example III.

Fig. 6 shows the variation of NOx conversion rate with NH ₃ /NO molar ratio when using the catalyst prepared in Example IV.

BEST MODE FOR CARRYING OUT THE INVENTION

According to the present invention, NOx is removed from exhaust gases in the presence of natural manganese ore. The average chemical composition and physical properties of natural manganese ores used as catalysts are listed in Tables 2 and 3.

Table 2

Average chemical composition of natural manganese ores components mn SiO ₂ Al ₂ O ₃ Fe CaO MgO _O2 balance of Mn&Fe %weight 51.83 3.13 2.51 3.86 0.11 0.25 38.11

O ₂ represents the total amount of manganese and iron because both manganese and iron co-exist in MnO ₂ , Mn ₂ O ₃ , Mn ₃ O ₄ , Fe ₂ O ₃ and Fe ₃ O ₄ , and as a result it is difficult to explain their respective compositions. It amounts to 80% by weight or more when calculated as _MnO2 .

It should be noted that the term "natural manganese ore" as used in this application refers to manganese ore found in mineral deposits on the earth's surface. As shown in Table 2, natural manganese ores are mainly composed of oxides of manganese, iron, calcium, magnesium, aluminum and silicon, with the highest content being manganese. In natural manganese ores, 80% by weight or more of manganese oxide is pyrolusite (MnO ₂ ).

table 3

Average Physical Properties of Natural Manganese Ore Density(kg/ ^m3 ) 3980 Pore volume ( ^cm3 /g) 0.0392 (5-3000 Angstroms) Surface area ( ^m2 /g) 11.0

The data in Table 2 show that in addition to manganese and iron, natural manganese ore contains various metal oxides known to be catalytic in SCR, so that it can be used as an SCR catalyst.

Pass the mixed gas of NOx, ammonia and oxygen into a reactor (such as a fixed bed reactor) using natural manganese ore as a catalyst, and then observe the conversion rate of NOx, which shows that at a relatively low temperature (about 150°C), natural Manganese ore has the maximum NOx conversion rate, and natural manganese ore can maintain 90% or higher of its maximum conversion rate in a fairly wide temperature range (about 130-250° C.). Therefore, the use of natural manganese ore can bring significant economic benefits, since SRC technology can be carried out without heating the exhaust gas to high temperature. In addition, the wide temperature range in which natural manganese ore can handle NOx allows it to be used in various process conditions.

In the presence of the catalyst of the present invention, the ammonia to NOx concentration ratio is preferably 0.7-1.2. For example, if too low a concentration ratio is used, the activity of the catalyst results in too low efficiency. On the contrary, if the concentration ratio exceeds 1.2, it is necessary to increase the amount of catalyst used to prevent residual unreacted NH ₃ . Therefore, it is economically disadvantageous.

According to the present invention, natural manganese ore is crushed into particles of uniform size to enhance catalytic activity by increasing surface area. Its size depends on the type of catalyst used. For example, when natural manganese ore is used for a honeycomb structure, it is pulverized to have an average particle size of 1 micrometer or less. If the particle size of the powder exceeds 1 micron, it is difficult to form the powder into a slurry, and thus it is almost impossible to coat the powder on a honeycomb structure. Alternatively, natural manganese ore may be crushed to a particle size as long as the resulting particles are sufficient to function as a catalyst when packed in a reactor. In this case, the pulverized natural manganese ore needs to be completely dehydrated, for example at 103 °C, to prevent side reactions when the catalyst is catalyzed.

The method of coating natural manganese ore on the honeycomb structure is described in detail below.

First, the natural manganese ore is pulverized with a grinder to an average particle size of 1 micron or smaller.

The powder is then added to distilled water and mixed together to form a solution. The amount of powder is preferably about 20-50% by weight, calculated on the weight of water. For example, if the amount of the powder is less than 20% by weight relative to the weight of distilled water, the coating operation performed later cannot be completed in a short time. On the contrary, if the concentration exceeds 50% by weight, the viscosity of the solution is too high, making coating difficult.

The pH of the solution was then adjusted to 6.5-6.8 with acid under stirring. Illustrative, non-limiting examples of usable acids include sulfuric acid, hydrochloric acid, nitric acid and acetic acid, preferably nitric acid. When the pH value is lower than 6.5, the fine particles will be aggregated together and precipitation will occur. Conversely, if the pH exceeds 8.5, ionic interactions occur between the fine particles of the solution. This effect can hinder the application of the solution.

Based on 100 parts by weight of the solution, about 1-5 parts by weight of binder is added. The binding agent can be selected from methoxymethyl cellulose (MC), polyvinyl alcohol (PVA), carboxymethyl cellulose (CMC), polyethylene glycol (PEG), silica gel, alumina sol and their mixture.

Next, a commercially available honeycomb structure was immersed in the solution for 2-3 hours, and then dried at room temperature. A further step of drying at 103°C for 4-6 hours is very beneficial to prevent side reactions when the resulting honeycomb structure is used as a catalyst. Then the honeycomb structure is baked in an electric furnace at 350-500° C. for 4-8 hours, so that the natural manganese ore powder is coated on it.

The present invention can be better understood with reference to the following examples, but these examples are only for illustration of the present invention and should not be construed as limiting the present invention.

Example I

This example tests the catalytic activity of natural manganese ore for reducing NOx. For this purpose, natural manganese ore was crushed into 40-50 mesh (average particle size 0.359 mm) granules, and these granules were filled into a fixed-bed reactor with an inner diameter of 8 mm to 3 ml. NOx at a concentration of 690 ppm was fed, and ammonia at a concentration 1.12 times the NOx concentration was fed at the same time. The gas is passed through the catalyst layer at a space velocity of 20000 h ^-1 .

Referring to Fig. 1, the data of Example 1 are plotted. These data show that the conversion rate of the catalyst obtained from pulverized natural manganese ore is close to 100% at around 150°C. This indicates that manganese oxide can be used as a catalyst with excellent NOx reduction performance. These data also show that the manganese ore conversion is maintained at 100% over a wide temperature range of 150-250°C. It has also been found that in this temperature range excess ammonia is completely oxidized away without leaving unreacted ammonia. This is considered to be caused by the low-temperature reducing ability of manganese oxide and the influence of other metal oxides contained in the ore or the synergistic effect between them. Therefore, it is not unreasonable to use natural manganese ore as SCR catalyst, at least 90% of NOx can be completely removed in the temperature range of 130-250 °C. Natural manganese ore, which is newly identified as a low temperature catalyst in the present invention, is preferably used for converting NOx at a temperature of 130-220°C.

Referring to FIG. 2, it shows the effect of _O2 concentration on the NOx conversion rate of the catalyst in Example I at predetermined temperatures (175°C and 200°C). To study this effect, the concentration of NOx was determined to be 430 ppm, the concentration of ammonia was determined to be 1.13 times greater, and the space velocity in the catalyst bed was determined to be 50000 h ^-1 . The data in Figure 2 show that oxygen concentrations at or above 0.5% have no effect on conversion. Since the oxygen concentration in the exhaust gas is 1% or higher on average, the catalyst of the present invention can exert its full catalytic reduction ability of NOx regardless of the oxygen concentration.

Referring to Figure 3, NOx conversion is plotted against temperature at various space velocities (GHSV). For this purpose, the concentration of oxygen was 3%, the concentration of NOx was 430ppm, and the supply amount of ammonia was 1.13 times that of NOx. As shown in this figure, the catalysts of the present invention have high efficiencies at lower temperatures even at space velocities as high as 70,000 hr ^-1 . Therefore, the catalysts of the present invention are not greatly affected by space velocity.

Example II

SCR technology is used to remove NOx by using a honeycomb structure coated with natural manganese ore powder.

To coat the honeycomb structure with the powder, natural manganese ore is first pulverized into a powder with an average particle size of 1 micron or less. This powder was added to 1000 grams of water to form a 30% by weight solution. While stirring the solution, it was adjusted to pH around 7 with nitric acid, and then 30 g of methylcellulose (MC) was added to the solution. A honeycomb structure preferably made of cordierite is immersed in the solution for about 3 hours, dried at room temperature, then dried at about 103°C for 5 hours, and baked in an electric furnace at a temperature of 400°C for 6 hours.

Before performing the SCR technique, the prepared honeycomb structure was inserted into a conical honeycomb reactor with a diameter of 5 cm. In this experiment, the oxygen concentration was 3%, the NOx concentration was 420 ppm, and the ammonia concentration was 1.10 times the NOx concentration. The diameter to height ratio of the honeycomb structure was 0.75.

Referring to FIG. 4, the catalytic activity of the catalyst of the present invention supported on the honeycomb structure is represented by the conversion rate of NOx and the emission of NH ₃ . It can be seen from the data in Fig. 4 that the catalyst of the present invention loaded on the honeycomb structure can remove Nox with high efficiency and does not emit ammonia.

Example III

Repeat the steps of Example II, except that the addition of natural manganese ore powder in water is 30% by weight, 40% by weight and 47% by weight, and the height of the honeycomb structure used is 13 mm, and the ratio of diameter to height is 0.25.

Referring to Figure 5, the NOx conversion as a function of the concentration of the catalytically active component in the solution coated on the honeycomb structure is plotted versus temperature. As shown in Figure 5, in order to maintain high catalytic activity of the catalyst supported by the honeycomb structure, the natural manganese ore component in the solution must reach a certain concentration. That is to say, in order to remove NOx efficiently, it is necessary to coat a certain amount of catalytically active components in natural manganese ore on the honeycomb structure. In fact, the conversion increased by 2-3% per application, up to a certain number of times (about 5 times).

Example IV

The natural manganese ore that is pulverized into an average particle size of 359 microns is filled in the same fixed-bed reactor as in Example 1, and this reactor is used to measure the change of the NOx conversion rate with the NH ₃ /NO molar ratio, and the conditions used are NO concentration of 440ppm flows through the reactor, the concentration of oxygen is 3%, and the reaction temperature is 200°C. The results are shown in Figure 6.

The data in Fig. 6 show that the conversion of NOx is almost proportional to the molar ratio of NH ₃ /NO before the molar ratio reaches 1:1, and the reaction of NH ₃ and NO is almost in the molar ratio of 1:1. When the molar ratio reaches 0.7, the NOx conversion rate of the catalyst begins to increase slightly, and when the molar ratio is 1:1, the NOx conversion rate of the catalyst reaches 100%. Therefore, the optimal condition for the molar ratio of _NH3 to NO is 0.7–1.2.

industrial application

In the presence of the above-mentioned catalyst of the present invention made of natural manganese ore, the SCR process using ammonia as a reducing agent can completely remove NOx from exhaust gas at a relatively low temperature of 130-250°C without discharging unreacted ammonia . Therefore, the catalyst of the present invention has excellent catalytic activity to convert NOx in exhaust gas even in a relatively low temperature range, and has excellent economic efficiency and can prevent adverse effects of ammonia emission.

The invention has now been illustratively described. It is to be understood that the terminology used is descriptive only and not restrictive. Many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that the present invention may be practiced within the scope of the claims and not only within the scope of the foregoing detailed description.

Claims (5)

1. A method for removing nitrogen oxides from waste gas, characterized in that nitrogen oxides are selectively catalytically reduced with ammonia as a reducing agent in the presence of a catalyst made from pyrolusite type natural manganese ore. 2. The method according to claim 1, characterized in that the catalyst is the pulverized natural manganese ore granular material filled in the reactor or coated on the honeycomb structure with an average particle size of 1 micron or less Natural manganese ore powder material. 3. The method of claim 2, wherein said honeycomb structure is immersed in an aqueous solution containing about 20-50% by weight of said powder and adjusted to pH6.5- 8.5. 4. The method according to claim 1, characterized in that said selective catalytic reduction technology is carried out at 130-250°C. 5. The method of claim 1, wherein said ammonia is supplied in a molar ratio of 0.7-1.2 relative to said nitrogen oxides.

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